This invention relates to improvements in the manufacture of so-called “MB cap sheets,” which are used for building bituminous membrane roofing systems, i.e., roofing systems composed of one or more layers of a water-proof membrane commonly made for a bituminous material or analog.
As well known in industry, this type of roofing system is typically used for covering “flat” roofs, i.e., roofs which are arranged in a horizontal configuration or a “low slope” roofs, i.e., roofs which are substantially horizontal but sloped enough to allow positive drainage.
When a new bituminous membrane roofing system is being built, the underlayment of the roofing system is normally formed by applying a liquid bituminous composition, e.g., molten bitumen or solvent based bitumen adhesive, to the roof substrate and then applying a pre-formed bituminous membrane over the bituminous composition. This can be done a single time, or multiple times thereby forming a “multi-ply” or “built-up” or “BUR” underlayment. The pre-formed bituminous membranes used for this purpose are usually composed of a carrier such as fiberglass, polyester or felt which is impregnated with bitumen or other similar material.
Once the underlayment is formed, the outermost layer of the roofing system is then applied to complete the bituminous membrane roofing system. As well known in the art, three different approaches are commonly used to form the outermost surface layer of a bituminous membrane roofing system.
In the first, referred to here as the “in-situ aggregate” approach, a layer of bitumen (asphalt) or other adhesive is laid down on the underlayment of the roofing system, i.e., the portion of the roofing system under the outermost layer, and a layer of aggregate typically on the order of about ⅜ inch (˜1 cm) in diameter is laid down on and partially embedded in the adhesive.
In the second, referred to here as the “liquid applied membrane” approach, the outermost surface layer is formed by coating the underlayment with a roof coating or covering which forms a relatively thick (i.e., membrane-like) waterproof coating.
In the third, referred to here as the “cap-sheet” approach, the outermost surface layer is formed from multiple preformed membranes or sheets (hereinafter “cap-sheets”) which are laid up or arranged in an adjacent, overlapping edge fashion (like shingles in a shingle roof) to cover the entire roof surface to be covered. A layer of bitumen (asphalt) or other adhesive is typically used to secure these cap-sheets to one another as well as the underlayment. Typically, these cap-sheets are made from a fibrous web or sheet of fiberglass, polyester fiber or both impregnated with bitumen (asphalt) waterproofing composition. A suitable aggregate is embedded in the asphalt adhesive during manufacture for adding desired color and UV protection for longer service life.
High end cap sheets, which are also referred to as MB cap sheets, are made from polymer modified asphalt coatings which exhibit low temperature flexibility for providing improved low temperature cracking resistance. However, a common problem that occurs in MB cap sheets if any moisture is present in or on the underlayment when these cap sheets are laid down is “blistering” in which small pimples or blisters appear on the surface of the MB cap sheet over time, especially when it has been subjected to wide swings in ambient temperature.